Method of forming ceramic bricks

ABSTRACT

A method for forming free standing ceramic bricks for use as tritium breeder material is disclosed. Aqueous solutions of sodium carbonate and potassium carbonate are mixed with an organic hydrocolloid dispersion and powdered lithium carbonate, spray dried, and ceramic bricks formed by molding in a die and firing.

CONTRACTUAL ORIGIN OF THE INVENTION

The U.S. Government has rights in this invention pursuant to ContractNo. W-31-109-ENG-38 between the U.S. Department of Energy and theUniversity of Chicago, the operator of Argonne National Laboratory.

BACKGROUND OF THE INVENTION

This invention relates to a process for the formation of ceramic bricksand more particularly for the formation of freestanding sintered bricksof lithium carbonate.

Lithium containing ceramics, such as LiAlO₂, are generally used as atritium breeder material in a breeder blanket in nuclear fusiontechnology. Lithium nuclei are split by high energy neutrons yieldingtritium, an isotope of hydrogen. A spatially uniform distribution oflithium isotopes reduces the complexity of many mathematicalcalculations concerning the dispersion of neutrons and the production oftritium. In addition, such ceramics can be used as neutron absorbingmaterial where appropriate. Lithium carbonate is the most readilyavailable lithium-containing ceramic. In the past, such lithiumcontaining ceramics have been used in particle form or packed in variouscontainers. The cost involved in the utilization of the ceramics inparticle form is significant and in particle form the ceramic can have afairly inefficient packing or density. It is essential that breedermaterials be distributed as uniformly as possible, thereby requiringminimal variations in packing, if discrete particles are used, a factorwhich can vary significantly in individual applications. If bricks areused, the bricks may not exhibit any striations or other disruptiveimperfections and variations in content. In addition, when forming thebricks, the green strength must be sufficient to permit handling of thegreen brick without a loss of integrity.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is an improved methodfor forming a lithium carbonate freestanding ceramic brick for use as atritium breeder material.

Another object of the subject invention is a method of cold pressing andsintering said ceramic brick, employing an organic binder material tomaintain the unsintered brick in the desired shape until the brick issintered.

A further object of the subject invention is the use of the sinteringaids sodium carbonate and potassium carbonate in combination with theorganic binder and lithium carbonate for the provision of a freestandingceramic brick of high density.

In accordance with the above objects, the subject invention provides fora method of forming such a freestanding ceramic brick through the stepsof preparing an aqueous solution of sodium carbonate and potassiumcarbonate, preparing an aqueous dispersion of a water dispersibleorganic hydrocolloid, and mixing both until a full solution is providedin the first and a uniform dispersion is provided in the second. Thesolution is mixed with the dispersion until a uniform consistency isachieved, at which point a fine lithium carbonate powder is added toform a smooth, consistent white slip. The water is removed from thecombined mixture such as by spray drying and bricks formed from theresulting powder by pressing and sintering. The resulting green brickmay be handled without a loss in integrity. The sintered bricks areshaped by sanding or filing to a desired geometric shape and are readyfor use. No striation or other non-uniform distribution of itscomposition in the sintered brick is observed.

DETAILED DESCRIPTION OF THE INVENTION

In the production of the freestanding brick according to the subjectinvention, the water dispersible organic hydrocolloid for use as anorganic binder of the green brick is preferably a blend of 85%microcrystalline cellulose and 15% sodium carboxymethylcellulose.

The hydrocolloid is used to bind the ceramic particles in the green,unsintered state, thereby holding the brick's shape until sintered, whenthe organic portion is essentially burned out. A preferred waterdispersible hydrocolloid may be procured as FMC Avicel Type CL-611. TheAvicel hydrocolloid binder is added to water to form a uniformdispersion. Sufficient hydrocolloid was added to form a 2-4% by weightof the total solution. An aqueous solution containing a mixture ofalkali carbonates other than lithium carbonate, and preferablyapproximately 10% each of Na₂ CO₃ and K₂ CO₃, is prepared. Thedispersion and solution are mixed, lithium carbonate is added withcontinuous stirring and the slip is then dried. In a preferred method ofdrying, the slip is run through a spray dryer such as a Buchi spraydryer, which is a small, laboratory-size spray dryer, which sucks up thesolution and blows it out through a two fluid nozzle. The resultingspray is mixed with heated air adjusted to an outlet temperature of 100°C.±5° C., as known in the art. Other spray dryers are contemplated foruse in the subject invention and may have a two fluid nozzle or acentrifugal nozzle; the solution may be sucked into the spray dryer orforced through by the application of pressure: further, the drying airmay be heated by gas or electricity. The resulting powder should have amoisture content of approximately 0.5-2%.

The dried powder is then pressed into a desired shape at approximately1,000-10,000 psi, and placed in an oven where it is fired, attemperatures increasing 50° C./hr. to a temperature between 385° C. and450° C. where it is held for 4 hours.

A fine particle size of Li₂ CO₃ may be used such as may be achieved bysieving the Li₂ CO₃ through a 100 mesh screen. A fine powder such asthis will provide a higher density in the sintered brick. It has beenfound that utilization of a coarse powder, i.e. that passing through ascreen of approximately 60 mesh, will result in a higher density in thegreen or unsintered brick. However, the fine powder (100 mesh) Li₂ CO₃is preferred both for its facility in spray drying and the higherdensity in the fired brick.

The following examples relate to the subject method of formingfreestanding ceramic bricks.

EXAMPLE Solution 1

30 grams distilled water

3.5 grams Na₂ CO₃

3.5 grams K₂ CO₃

The above ingredients are mixed until full solution is obtained; mixingis continued.

Dispersion 2

70 grams distilled water

4.0 grams Avicel

The above materials are mixed. White globs are formed at first but aftera few minutes, the globs go away and a smooth dispersion of eggwhiteconsistency is attained. Mixing is continued. The solution is added tothe dispersion and mixing continued. 93.0 grams of Li₂ CO₃ fine powder(100 mesh) is added to the combination and mixed until a smoothconsistent white slip is obtained. The slip is then spray dried at 100°C. outlet temperature utilizing a Buchi spray dryer. The resulting spraydried powder has a moisture content of 1.5%. A green brick is pressed at6,000 pounds (1,100 psi) using this spray dried powder in a die press.The resulting green brick satisfactorily holds its shape during handlingand has a density equal to approximately 60% of theoretical. The greenbrick was sintered by heating the brick to 450° C. at the rate of 50° C.per hour, holding at 450° C. for four hours and then allowing thesintered brick to cool to room temperature. The resulting density of thesintered brick was 88%. The sintered brick was slightly warped, and nostriations or other non-uniform distribution of its composition wasobserved. After being sanded to the necessary specified size and coatedwith a tritium barrier comprising an epoxy paint to make the resultingbrick impermeable to water vapor and other gases, the sintered brick wasready for use as a tritium breeder material.

During the sintering phase, the binder is vaporized or oxydized; aeutectic mixture of the sintering aids, i.e. the sodium carbonate andpotassium carbonate, with a small portion of lithium carbonate, areliquified, causing the lithium carbonate particles to consolidate. Theresultant brick is lithium carbonate grains with a grain boundary phaseof lithium carbonate with sodium carbonate and/or potassium carbonate.

As shown in the above example, a sintered lithium carbonate brick whichmay be formed in a specific reproducible size and shape. The brick maybe handled in the green unsintered state, as a result of the greenstrength provided by the organic hydrocolloid binder material. As aresult of the improved handling properties thereby imparted in the greenstate, the green brick may be more easily molded and sintered, whileretaining a substantially uniform distribution of its composition foruse as a tritium breeder material.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of formingfree-standing ceramic bricks comprising the steps of:mixing an aqueoussolution of a mixture of alkali carbonates selected from the groupconsisting of K₂ CO₃ and Na₂ CO₃ with an aqueous dispersion of anorganic hydrocolloid to form a second aqueous mixture, blending inpowdered lithium carbonate to form a slip, drying said slip to form apowder, molding said powder to form a green brick, and firing said greenbrick to form a sintered brick.
 2. The method of claim 1 wherein saidslip is dried to result in a powder having 0.5% to 2.0% moisturecontent.
 3. The method of claim 1 wherein said powder is molded at1000-10,000 psi.
 4. The method of claim 1 wherein said green brick isfired by firing in an oven having the temperature raised at 50° C./hr.to a temperature between 385° C. and 450° C., where it is kept for 4hours.
 5. A method of forming free-standing ceramic bricks comprisingthe steps of:mixing an aqueous solution of a mixture of alkalicarbonates selected from the group consisting of K₂ CO₃ and Na₂ CO₃ withan aqueous dispersion of an organic hydrocolloid to form a aqueousmixture, blending powdered lithium carbonate into said mixture to form aslip, spray-drying said slip to form a powder of 0.5% to 2.0% moisturecontent, molding said powder to form a green brick, and firing saidgreen brick in an oven by raising the temperature of the oven 50° C./hr.until a temperature between 385° C. to 450° C. is reached and holdingfor 4 hours to form a sintered brick.
 6. The method of claim 5 whereinsaid organic hydrocolloid comprises a blend of microcrystallinecellulose and sodium carboxymethylcellulose.
 7. The method of claim 7wherein said blend comprises 85% microcrystalline cellulose and 15%sodium carboxymethylcellulose.
 8. A method of forming free-standingceramic bricks for use as neutron absorbing material comprising thesteps of:mixing an aqueous solution of a mixture of alkali carbonatesselected from the group consisting of K₂ CO₃ and Na₂ CO₃ with an aqueousdispersion of a blend of 85% microcrystalline cellulose and 15% sodiumcarboxymethylcellulose to form an aqueous mixture, blending powderedlithium carbonate into said mixture to form a slip, spray-drying saidslip to form a powder of 0.5% to 2.0% moisture content, molding saidpowder at 1,000 to 10,00 psi to form a free-standing green brick, andfiring said green brick in an oven by raising the temperature of theoven 50° C./hr. until a temperature between 385° C. to 450° C. isreached and holding for 4 hours to form a sintered brick.